The occurrence of Venous Air Emboli (VAE) has been well documented in the medical literature. VAE has been shown to be a significant cause of mortality and morbidity in many different types of surgical procedures.
Venous Air Emboli occur when air bubbles enter the blood stream secondary to a negative pressure gradient. Negative pressure gradients can occur any time the surgical site is above the level of the right atrium. When air enters the venous system, small bubbles are formed. These bubbles travel to the right side of the heart, enter the right atrium, pass through the tricuspid valve into the right ventricle, through the pulmonic valve and into the pulmonary artery. As the bubbles traverse the pulmonary capillary tree, they become wedged in the distal pulmonary micro-vascular system. This results in increased pulmonary deadspace.
When gone undetected for sustained periods of time, it is possible for some bubbles to completely traverse the pulmonary capillary tree. These bubbles enter the left side of the heart and ultimately the arterial blood flow. Once inside the arterial circulation, the potential arises for severe intra-operative and/or post operative cardiopulmonary and neurological complications.
Precordial doppler has been shown to be a very specific and accurate detector of VAE. However, there are many problems associated with doppler monitoring. Positioning of the doppler probe over the correct intercostal space, electro-surgical interference, heart murmurs and the requirement of a trained ear make doppler monitoring less than adequate for VAE detection.
Continuous CO.sub.2 monitoring is a sensitive indicator of VAE. However, CO.sub.2 monitoring is very non-specific for VAE. There are many different physiological alterations that lower CO.sub.2 values. Metabolic changes, cardiac output changes, hyperventilation and deadspace changes can lower expired CO.sub.2 values.
Many studies have been performed to analyze conditions and parameters indicative of VAE. Several of these studies are summarized below.
Matjasko et al. (Matjasko, Petrozza and Mackenzie, Sensitivity of End-tidal Nitrogen in Venous Air Embolism Detection in Dogs. Anesthesiology 63:418-423, 1985) describes a study done to compare the sensitivity for the detection of venous air emboli of end-tidal nitrogen (EtN.sub.2) monitoring, precordial Doppler (PD), end-tidal CO.sub.2 (EtCO.sub.2) and pulmonary artery pressure (PAP). This study concluded that: 1) precordial Doppler monitoring is the most sensitive qualitative detector of air entry into the superior vena cava and heart, however, it does not allow quantitative measurement of air entry into and dissipation from the lungs; and 2) changes in EtN.sub.2 following low-dose infusion VAE are less sensitive than changes in EtCO.sub.2, while during bolus VAE they are equally sensitive. Additionally, they expressed concern that shared operating room mass spectrometers would be sensitive enough and sample frequently enough to be of value in the early detection of clinical VAE.
Matjasko et al. (Matjasko, Hellman and Mackenzie, Venous Air Embolism, Hypotension, and End-tidal Nitrogen. Neurosurgery 21:378-382, 1987) describes a study done to compare the sensitivity for the detection of venous air emboli of end-tidal nitrogen (EtN.sub.2) monitoring and end-tidal CO.sub.2 (EtCO.sub.2) monitoring during large bolus and infusion VAE.
Mangan et al. (Mangan, Boucek, Powers and Shadduck, Rapid Detection of Venous Air Embolism by Mass Spectrometry during Bone Marrow Harvesting. Exp. Hematol. 13:639-640, 1985) describes the detection of a venous air embolism by mass spectrometric monitoring of the patient's expired gases. They found that air emboli are suspected when end-tidal CO.sub.2 decreases (down 1.5%) and also when expired nitrogen increases (up 4%).
Russell et al. (Russell, Richard and Snider, Detection of Venous Air Embolism in Dogs by Emission Spectrometry. J Clin Monit. 6:18-23, 1990) describes the detection of a venous air embolism by emission spectrometry monitoring of the end-tidal nitrogen and compares the results with monitoring by mass spectrometry.
Prager et al. (Prager, Gregory, Ascher and Roberts, Massive Venous Air Embolism during Orthotopic Liver Transplantation. Anesthesiology 72:198-200, 1990) reports the detection of a venous air embolism by mass spectrometric monitoring of the patient's end-tidal CO.sub.2 and end-tidal expired nitrogen.
Frankel et al. (Frankel and Holzman, Air Embolism During Posterior Spinal Fusion. Can J Anaesth 35:5:511-514, 1988) presents a list of methods available for the detection of VAE including: precordial doppler, oesophageal stethoscope, measurements of end-tidal CO.sub.2, end-tidal nitrogen, pulmonary artery pressure, transoesophageal echocardiography and transcutaneous O.sub.2 and CO.sub.2 monitoring.
Lew et al. (Lew, Tay and Thomas, Venous Air Embolism During Cesarean Section More Common Than Previously Thought. Anesth Analg 77:448-452, 1993) reports the use of a Raman scattering analyzer to continuously monitor inspired and expired nitrogen, oxygen, halothane, nitrous oxide, and end-tidal carbon dioxide levels via a side stream sampling catheter. Positive evidence of air embolism was defined as an increase in expired nitrogen concentrations of 0.1 vol % from the baseline.
Drummond et al. (Drummond, Prutow and Scheller, A Comparison of the Sensitivity of Pulmonary Artery Pressure, End-Tidal Carbon Dioxide, and End-Tidal Nitrogen in the Detection of Venous Air Embolism in the Dog. Anesth Analg 64:688-692, 1985) reports the results of an experiment seeking to define the relative sensitivities of end-tidal CO.sub.2 analysis, end-tidal nitrogen analysis and pulmonary artery pressure (PAP) monitoring in the detection of VAE. They conclude that, where the capacity to identify increases in expired nitrogen on the order of 0.04% can be achieved, end-tidal nitrogen monitoring will identify VAE events with a sensitivity similar to that of PAP and EtCO.sub.2. They also note that the difficulties inherent in achieving this level of nitrogen sensitivity represents a major limitation in the application of this test.
English et al. (English, Westenskow, Hodges and Stanley, Comparison of Venous Air Embolism Monitoring Methods in Supine Dogs. Anesthesiology 48:425-429, 1978) compares a variety of VAE monitoring methods for the detection of VAE including: precordial Doppler ultrasound frequency; end-tidal CO.sub.2 concentration; mean pulmonary arterial, central venous and arterial pressures; esophageal heart sounds, including high-pitched barely audible tinkling sounds, enhanced second heart sound, systolic and mill-wheel murmurs; the electrocardiogram; arterial blood carbon dioxide and oxygen tensions; heart rate and the presence of cardiac arrhythmias.
None of these studies or the methods described therein disclose a monitoring system that automatically detects and signals the presence of a VAE. For example, the precordial Doppler techniques requires that a trained individual continuously monitor the output of the instrument and interpret the output. Likewise, while it is generally recognized that an increase in the end-tidal nitrogen concentration may be indicative of a VAE, or that a decrease in the end-tidal CO.sub.2 concentration may be indicative of a VAE, neither is considered to be a reliable indicator. Additionally, as with the precordial Doppler, monitoring of end-tidal nitrogen and/or end-tidal CO.sub.2 concentrations requires the attention of an operator to observe the behavior of these parameters and determine when changes therein are significant and whether they are indicative of a VAE or some other factor.
Thus, the ideal monitor for detection of venous air embolism should be: noninvasive; exquisitely sensitive; reliable; automatic; specific for air-induced changes; quantitatively reflect the size of the embolus; and have a duration of positive response accurately reflecting embolus onset and resolution. No single VAE monitor presently available meets all of these requirements.